The purpose of this research is to contribute to our understanding of the neuronal machinery responsible for learning and response selection in complex, integrated behavioral repertoires. These related problems are to be studied in a relatively simple system, the lateral giant escape reaction circuit of the crayfish, which is subject to a variety of forms of control and modulation. Behavioral, electrophysiological, and anatomical research is proposed on (1) the cellular basis of sensitization, long-term potentiation, and associative learning, (2) modulatory circuitry that is involved in integrating lateral giant escape into the behavioral repertoire of the animal, and (3) effects of neural activity on synapse formation during regeneration following injury.